This invention relates to the field of piezoelectric material testing, and is concerned with evaluating the physical and electrical properties of piezoelectric materials. The invention also relates to the selection of desired fragments from a sample lithium niobate piezoelectric material for satisfactory manufacture of surface acoustic wave (SAW) signal delay lines.
In manufacturing SAW delay lines presently accepted practice contemplates the fabrication of one or more test delay lines on a sample of substrate material taken from each production lot that is considered for manufacturing use; the successful performance of these test delay lines is assumed to predict satisfactory properties in the substrate material for larger quantity manufacture of the SAW delay line devices. A further practice in this current manufacturing is to assume that if piezoelectric wafers pass through the manufacturing steps of electrically poling, sawing into wafers, and surface polishing without incurring fracture or other physical damage, then the wafers came from a desirable boule of substrate material. In essence, this boot strap selection process relies on accumulated experience with wafers from a particular boule for determining suitability of that boule for continued use.
The importance of signal delay lines in signal correlation and signal processing applications both in military and commercial electronic equipment clearly suggests that improved testing and selection for delay line substrate materials are needed. The improvement of the boot-strap boule testing described above, will significantly advance the art of delay line manufacture and give rise to decreased cost, increased reliability, and increased predictability of manufactured device characteristics.
The patent of Henry M. Gerard, U.S. Pat. No. 3,840,825, discloses a coupling arrangement for communicating surface acoustic wave energy between adjacent bodies of propagating material. The Gerrard patent contemplates the fabrication of SAW delay line devices from two different materials in order that the desirable properties of each contribute to the delay line characteristics properties minimal compromise by the less desirable properties. In the Gerrard invention, an acoustic wave guide film is used to conduct SAW energy around the epoxy resin adhesive used to join two different substrate materials. The acoustic wave guide film of Gerard is fabricated from such materials as gold, aluminum or nickel which have desirable slow SAW propagating characteristics. This film is selected in thickness and shape to optimize SAW energy transfer and minimize loss and reflection problems.
The patent of Sygmond Turski, U.S. Pat. No. 3,959,747, discloses a surface acoustic wave device fabricated on a lithium niobate substrate and employing a particular metallization and bonding pad fabrication.
The patent of Shusuke Ono, U.S. Pat. No. 4,236,095, discloses a surface acoustic wave structure which also employs two different acoustic materials and an acoustic coupling between the materials. The Ono patent is incorporated by reference herein for, inter alia, its discussion of acoustic coupling technology.